Editor: Optimize Inserter props generation and reconciliation

[aduth]

Related (potentially blocks): #11018

This pull request seeks to apply minor refactoring to the Inserter to optimize its rendering:

• Avoid passing selectedBlock to the root component, as its reference changes frequently (i.e. every keystroke in the selected paragraph) and is only needed for the opened menu.
  • Similarly, avoid passing items array to the root component, and pass only the simple boolean reference of hasItems for use in considering whether the inserter should be rendered at all (via ifCondition)
• Assign toggle and content renderers as a consistent bound reference on the root component class, avoiding reconciliation which would otherwise occur in the changing function references on each render.

Further optimization could be achieved in simplifying how hasItems is calculated, since getInserterItems is easily the most complex / convoluted selector we have in the core/editor store. This has not been done here, but I foresee one or more of:

• Splitting getInserterItems into more granular memoized selectors
• Creating a separate hasInserterItems selector which performs the more simple filtering (not mapping) operations
• Defer the internal "build" operations of getInserterItems to the inserter component itself, passing instead the minimal raw data necessary.

Testing instructions:

Verify there are no regressions in the behavior of the inserter, with notable variations:

• With and without a block selected
• Inserter in header and empty-paragraph-adjacent

Using React DevTools "Highlight Updates" option, verify that the Inserter does not re-render when typing within a paragraph.

[gziolo]

I no longer see re-renders when navigating between blocks. Code is very identical to my previous implementation but this one is backward compatible so this is a big plus.

Nice work! 💯